The present invention is related to the field of optical communications, and more particularly to optical modulation in optical communications systems.
The ever-growing demand for bandwidth of optical transmission systems requires an efficient utilization of available optical spectral bandwidth of the systems. Spectral efficiency can be expressed as the ratio of the bit-rate of an optical channel divided by the frequency spacing between channels in a wavelength-division-multiplexed (WDM) system. As an example of such a measure, current commercial systems exhibit a per-channel bit-rate of 10 Gb/s and channel spacing of 50 GHz, which provides a spectral efficiency of 0.2 bits/s/Hz.
Traditionally, optical communications have employed so-called “non-return-to-zero” or NRZ amplitude modulation of an optical carrier signal. In NRZ modulation, the optical carrier is either “on” or “off” for the duration of each signaling interval of the modulating data signal. While NRZ modulation in optical communications systems has the advantage of simplicity, it is known to suffer from relatively poor spectral efficiency.
It is known that more advanced modulation formats such as duo-binary formats (also referred to as “phase-shaped binary transmission” or PSBT), band-limited return-to-zero (RZ) format, and vestigial-side-band (VSB) format offer better spectral efficiency than traditional NRZ modulation. The spectral efficiencies of these formats can be as high as 0.8 bits/s/Hz without the use of polarization multiplexing.
Another class of more advanced modulation formats is known as “carrier-suppressed return-to-zero” (CS-RZ) modulation. CS-RZ modulated signals have the unique feature that adjacent pulses have a phase difference of π, which results in a suppressed spectral component at the central (carrier) frequency of the modulated optical signal. The use of such signals for high quality pulse train generation and for soliton compression has been proposed. It has also been discovered that CS signals generally are very robust with respect to nonlinear effects in optical fibers. Therefore, the use of the CS format for optical data transmission has also been suggested.